In order to construct an inverter comprising few switches with high-frequency clocking which is nevertheless able to shape a sinusoidal AC current, it is known to combine a DC/DC converter with an unfolding bridge. In this combination, the DC/DC converter shapes half-cycles of the AC current, and the unfolding bridge connects two AC current terminals of the inverter with alternating polarity to the outputs of the DC/DC converter or the poles of an interposed voltage link circuit. The reversal of the polarity is performed upon each zero crossing of an AC voltage applied to the AC current terminals externally. In general, this is the AC voltage of an AC grid into which electrical energy from a DC generator is fed by the inverter.
In order that such an inverter comprising a DC/DC converter and an unfolding bridge is provided with reactive power capability, various measures are known. These measures can be subdivided into two groups.
In the case of the measures in the first group, the current is output by the DC/DC converter only ever in the direction of the external AC voltage, but with a shift in the centroid of its current-time integral relative to the centroid of the voltage-time integral during each half-cycle of the AC voltage. This is described for example in DE 10 2010 035 020 A1. A unidirectional DC/DC converter is sufficient for realizing this measure. However, only small phase shift angles between the output AC current and external AC voltage can be realized expediently in this way. As the phase shift angle increases, the output AC current is increasingly deformed relative to the desired sinusoidal waveform and accordingly has a total harmonic distortion that increases with the phase shift angle. A high total harmonic distortion indicates that a signal has a pronounced harmonic component, which may result in electromagnetic interference.
In the case of the second group of measures, the DC/DC converter is implemented in bidirectional fashion, such that it may allow the current to flow in the opposite direction to the instantaneous voltage at the AC output as well. In this regard, DE 10 2009 029 387 A1 discloses an inverter, in particular a solar cell inverter of a photovoltaic installation, in which a semiconductor bridge circuit as polarity reverser is combined with a DC/DC converter in the form of a DC chopper controller designed as a four-quadrant controller. The DC chopper controller may comprise in particular a combination of a buck converter and a boost converter or a buck-boost converter with a common inductance.
During the operation of an inverter comprising a bidirectional DC/DC converter and an unfolding bridge for providing reactive power, i.e. in the case of a phase shift between AC current and AC voltage at the AC output of the inverter, however, the direction of the current flowing through the DC/DC converter has to be reversed when the unfolding bridge reverses the polarity between the half-cycles of the AC voltage. In the case of a reversal from a negative current, relative to the instantaneous voltage, to a positive current, this generally does not pose a problem because the DC input voltage of a DC source connected to the DC input acts as a driving force for this. The situation is different, however, in the case of a reversal from a positive current, in comparison with the instantaneous voltage at the AC output, to a negative current. This is accounted for by the only very low instantaneous voltage at the AC output at the zero crossing of the AC voltage. With this very low instantaneous voltage, a voltage-time integral required for reversing the direction of the current through an inductance of the DC/DC converter may be established only over a comparatively long time period. This time takes effect as a dip in the current and accordingly as a deviation from the desired sinusoidal profile of the AC output by the inverter. A high total harmonic distortion of the output AC current is the consequence.
This problem is not addressed in DE 10 2009 029 387 A1.
CN 103208935 A also discloses the combination of a bidirectional DC/DC converter with an unfolding bridge for providing reactive power.
In accordance with CH 700 030 B1, a bidirectional DC/DC converter combined with an unfolding bridge is subdivided into a buck converter and a boost converter, which have a symmetrical design with inductances between the two poles of an input terminal and a voltage link circuit, wherein these inductances are used both by the buck converter and by the boost converter. Here, too, there is no discussion of the high total harmonic distortion of the output AC current when providing reactive power.
WO 2013/134904 A1 describes an inverter comprising a buck-boost converter of symmetrical design and an unfolding bridge.
WO 2012/146414 A2 describes methods having the features of the preambles of independent patent claims 1 and 13 and an inverter having the features of the preamble of alternative independent patent claim 15. In this case, a bidirectional DC/DC converter in the form of a buck-boost converter is combined with an unfolding bridge to form an inverter with reactive power capability. In order to avoid difficulties in the case of changing the direction of the current flowing via the voltage link circuit in the case of a negative phase shift angle between AC current and AC voltage, i.e. in the case of an AC current lagging behind the AC voltage, a sudden change in the set point profile of the current through the inductor of the buck-boost converter is avoided. For this purpose, the set point profile of the current is modified such that it likewise has a zero crossing at zero crossing of the AC voltage. The resulting deviation of the output current from the desired sinusoidal profile results in a high total harmonic distortion here as well.
JP 2002 369388 A discloses the combination of a bidirectional boost converter with an inverter bridge in which, at least at the zero crossing of the voltage, at least two switches of the inverter bridge besides two switches of the boost converter are subjected to high-frequency clocking for the purpose of shaping the AC current. The inverter bridge is therefore not an unfolding bridge whose bridge switches are clocked substantially at a low frequency of the order of magnitude of the frequency of the output AC current.
WO 2011/042567 A1 discloses an inverter with reactive power capability which comprises two buck-boost converters, the inductors of which are respectively connected to one of the two terminals of an AC output. In this case, the two buck-boost converters are bridged alternately for a half-cycle of the output AC current. Reactive power with a phase shift angle of the output AC current deviating from zero is realized by blockwise outputting of pure active power and pure reactive power over in each case one period or a plurality of periods of the AC voltage.